The planographic printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data. Recently, a CTP system (hereinafter also referred to as thermal CTP) according to various systems employing infrared laser has been proposed.
As one embodiment of the thermal CTP, there is a process (so-called wet type thermal CTP) in which a printing plate material is developed with a liquid developer to form an image where solubility to the developer of the image formation layer of the printing plate material is varied by exposure. However, this process has problems in that an exclusive alkali developer for development is required like a conventional PS plate, developability varies due to the state (temperature or degree of fatigue) of the developer, which results in poor image reproducibility, or there are limitations to handling under room light.
A thermal processless CTP (including one capable of being developed on a press) has been developed which does not require specific development. The thermal processless CTP has been noted, since it can be applied to a printing press employing a direct imaging (DI) process in which an image is directly written on the press, followed by printing.
As one embodiment of the thermal processless CTP, there is a CTP of the ablation type. Examples of the CTP of the ablation type include those disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These references disclose a printing plate material comprising a support, and provided thereon, a hydrophilic layer and a lipophilic layer, either of which is an outermost layer. When a printing plate material is imagewise exposed in which the hydrophilic layer is an outermost layer and the lipophilic layer containing a light-to-heat conversion material is under the hydrophilic layer, the hydrophilic layer is imagewise ablated by heat generated in the lipophilic layer and removed to reveal the lipophilic layer, whereby an image is formed.
A printing plate material has been developed which is capable of forming an image without ablation, and does not require development treatment employing a special developer or wiping-off treatment. There is, for example, a CTP as disclosed in Japanese Publication Nos. 2938397 and 2938397, which comprises an image formation layer containing thermoplastic particles and a water-soluble binder and which is capable of be developed with a dampening solution on a printing press (on-press development). The CTP of this type, when a grained aluminum plate is used as a hydrophilic support, needs to incorporate a light-to-heat conversion material (generally colored) in the image formation layer and has problem of contaminating a printing press during on-press-development. Further, the CTP of this type needs to increase an amount of a light-to-heat conversion material in the image formation layer for high sensitization, which is sought as a processless CTP, but the increase is more likely to contaminate a printing press.
As a method for preventing the contamination of a printing press which is caused during on-press development, a method has been proposed which employs, as a hydrophilic support, a support in which a hydrophilic layer containing a light-to-heat conversion material is provided on a substrate. Such a hydrophilic layer makes it possible to eliminate the light-to-heat conversion material from the image formation layer. Further, this method can achieve high sensitization without contaminating a printing press, since it can increase the amount of the light-to-heat conversion material in the hydrophilic layer or incorporate a slight amount of the light-to-heat conversion material in the image formation layer.
However, a material for forming an irregularity structure on the surface of the hydrophilic layer is generally contained in the hydrophilic layer besides the light-to-heat conversion material in order to improve a printing property or an image retention property. For example, the present inventor discloses, in Japanese Patent O.P.I. Publication No. 2000-225780, a hydrophilic layer containing porous inorganic fillers with a particle diameter not more than 1.0 μm for improving a printing property or an image retention property besides the light-to-heat conversion material. Further, the present inventor discloses, in Japanese Patent O.P.I. Publication No. 2002-370465, a hydrophilic layer containing plural irregularity structure-forming inorganic fillers and an inorganic binder with high porosity as well as the light-to-heat conversion material in order to improve a printing property or an image retention property.
The hydrophilic layer described above has a good printing property and good image retention property, however, the hydrophilic layer has also problem in that the layer strength is relatively low, since materials constituting the hydrophilic layer are highly porous. When the amount of the light-to-heat conversion material in the hydrophilic layer is increased in order to increase sensitivity, it is necessary to decrease material as a binder by increment of the light-to-heat conversion material, which further lowers the layer strength. This lowers abrasion resistance of the hydrophilic layer during printing, and greatly lowers printing durability, particularly when printing conditions vary during printing, for example, printing pressure or ink roller pressure increases.
As the countermeasure, there is a method which decreases the inorganic fillers etc., by the increment of the light-to-heat conversion material, however, this method has problem in that a printing property and an image retention property deteriorate.
It has been considered that in a printing plate material comprising a hydrophilic layer containing a light-to-heat conversion material, it is extremely difficult to increase sensitivity or to improve or maintain printing properties or printing durability.